(1) Field of the Invention
The invention relates to the fabrication of semiconductor wafers, and more particularly to a method of eliminating the sticking of the wafer to the wafer clamp ring after wafer processing in excess of 120 Kilo Watt Hours (KWH's).
(2) Description of the Prior Art
It is required to secure a semiconductor wafer against movement during wafer processing. This securing or clamping of the semiconductor wafer assures the maintenance of repetitive and precise processing tolerances and thus insures improved wafer yields.
During Prior Art Physical Vapor Deposition (PVD) processing of the semiconductor wafer, a processing step used to deposit a metal film on the semiconductor wafer surface, a metal clamping ring 10, FIG. 1, is commonly used to secure the semiconductor wafer. Intense heat is generated in the clamping ring during this process due to the plasma generated during the process. This can lead to considerable expansion of the clamping ring. The area of contact between the clamping ring and the wafer must therefore be such that this expansion can be accommodated so that the inner diameter of the clamping does not equal or exceed the diameter of the wafer.
When processing a round wafer, a conventional wafer clamping arrangement, FIG. 1, secures wafer 12 to the wafer cooling pedestal 14 with a circular wafer clamping ring 10. The clamping ring 10 is used to press the edge of the wafer into the continuous sealing abutment with the upper surface of the wafer pedestal 14. A port 16 is provided to flow a supply of an inert coolant gas 18, such as argon, to the backside of the wafer to improve thermal transfer between the wafer and the heater chuck. This takes advantage of the large thermal mass of the heater chuck 14 relative to the wafer 12 for conducting temperature. In this way, a predictable and consistent temperature is maintained across the wafer surface during wafer processing, and the various process steps that are used to fabricate devices on the wafer surface may be carried out in a reliable manner.
During standard PVD processing, deposition of the metal film on the surface of the semiconductor wafer 12 results in deposition of a metal film on the surface of the clamping ring 10. This deposition alters the profile (height and inner diameter) of the clamping ring 10 which in turn results in the metal ring, that is its modified profile, being shadowed on the semiconductor wafer which is being processed. This shadowing has a negative effect on wafer yield and must therefore be restricted or eliminated.
During wafer processing electrical charges build up on the surface of the wafer between the wafer and the wafer pedestal. For instance, it is known that a negative charge can build up on back surface 11, FIG. 1, of the wafer 12 due to the presence of a DC plasma within the processing environment. The build up of such a charge on the wafer surface can have a serious detrimental effect since such a charge is of opposite polarity from the charge on the surface of the pedestal 14. This results in the wafer 12 sticking to the pedestal 14, removal of the wafer under these conditions can be time consuming and can result in wafer breakage or damage. The design of the clamping ring 10 is affected by this phenomena to the extent that the clamp ring 10, if it is properly manufactured using conductive material and if it is properly applied, can drain or eliminate the electrical charge between the back side 11 of the wafer 12 and the wafer pedestal 14 and in doing so help to prevent the wafer 12 from sticking to the wafer pedestal 14 due to electrical charges.
Also, the clamp ring arrangement which is currently used to clamp the wafer into the wafer processing tool encounters wafer sticking problems due to the build-up of extraneous material during the sputtering process in the wafer to wafer-clamp contact area 13. The accumulation of the extraneous material is a function of the length of the processing time and the power applied during the process, that is the process kilowatt-hours (KWH). The wafer-sticking problem limits the Process Kit (PK) time to 120 kWh on average.
Referring now more particularly to FIG. 2, this figure shows the Prior Art of wafer clamping. In this figure the wafer 12 is securely clamped down by the clamping ring 10 during wafer processing, the wafer clamping ring 10 has one cut-out or extrusion 22 with a cut-out width of 23 and cutout height of 24. Wafer 12 will maintain contact with clamping ring 10 at location 13 until the deposition is completed.
Typical dimensions for the cut-out sides are as follows: side 23 equals 1.5 mm.+-0.02 mm., side 24 equals 0.35 mm.+-0.02 mm.
The clamping ring 10 was examined after a problem of wafer sticking was observed; a layer of black film, which was deposited on the contact site, was analyzed. The average recorded Process Kit (PK) life up to the occurrence of the wafer sticking problem was 130 to 190 Kilo Watt Hours (kWh) for a 150 degrees Centigrade (C.) process and 110 kWh for a 500 degrees C. process.
U.S. Pat. No. 5,421,401 to Sherstinsky et al. teaches clamping a wafer to the wafer processing tool but limits itself to wafers having a flat portion while it has as objective to prevent leakage of coolant gases circulated at the back of the wafer into the process environment.
U.S. Pat. No. 5,467,220 to Xu teaches a wafer pedestal used to heat and cool a wafer during wafer processing by placing a yoke on top of the wafer clamp ring, this yoke provides a reflector which improves temperature uniformity across the wafer.
U.S. Pat. No. 5,460,703 to Nulman et al. teaches a clamping ring made of a thermally nonconductive material having a low thermal coefficient of expansion. Such material allows the production of a clamp ring having the largest possible diameter resulting in more wafer surface availability, reduced wafer shadowing due to metal film build-up on the clamp ring and improved wafer temperature uniformity.
U.S. Pat. No. 5,484,011 to Tepman et al. teaches a clamping ring with a temperature regulated platen for clamping a wafer to the platen but does not address the wafer-sticking problem.
U.S. Pat. No. 5,228,501 to Tepman et al. as above, for U.S. Pat. No. 5,1484,011.